Can't select any vertex in an object!
K_Horeman's answer helped me fix it:
https://blenderartists.org/t/noob-weird-selection-issues-cant-select-vertices-of-a-mesh-in-edit-mode/622979/2
&quot;If it’s selected in orange with no vertices when you’re in edit mode, it is a separate object. You’ll have to go to edit mode on that object. You can only be in edit mode on one object at a time.&quot;
I didn't have the correct mesh selected in the Viewer (top-right) before entering Edit mode. So I highlighted all 4 objects you see in the picture, and I can select their vertices.